Engines of the type in which a rocker arm is provided in a valve mechanism draw in and discharge a fuel gas and an exhaust gas by pressing a valve end, so as to open the valve with an adjustment screw on the distal end of a rocker arm that is actuated by a cam. When the rocker arm returns to an original position, the valve is closed again under a resilient force of a spring.
A clearance (hereinafter referred to as a tappet clearance) is provided between the valve end and the adjustment screw, for allowing the valve to be fully closed when the rocker arm returns to the original position. If the tappet clearance is too small, then the clearance may possibly be eliminated due to thermal expansion at high temperatures. If the tappet clearance is too large, then the valve end and the adjustment screw produce large sounds as noise when they contact each other. Therefore, the tappet clearance has to be adjusted accurately to an appropriate value (or within an appropriate range) that is preset in design. Particularly, a process for manufacturing a large quantity of engines in a wide variety of types needs to have a reduced adjustment time per engine, while maintaining a high adjustment accuracy level. It is preferable to be able to adjust the tappet clearance automatically in order to prevent adjustment fluctuations.
Processes for adjusting tappet clearance are disclosed in Japanese Patent Publication No. 62-8609, Japanese Laid-Open Patent Publication No. 11-153007, and Japanese Laid-Open Patent Publication No. 2001-27106. An adjustment apparatus used by the process disclosed in Japanese Patent Publication No. 62-8609 has an actuator for rotating a driver, a displacement measuring device for measuring displacement of a valve in directions in which the valve is opened and closed, and a means for engaging a rocker arm to press a pad surface of the rocker arm against a cam surface. The means for engaging the rocker arm has a pressing lever element for pressing the pad surface against the cam surface under strong forces. The pressing lever element presses the pad surface reliably against the cam surface for increased adjustment accuracy.
According to the process described in Japanese Patent Publication No. 11-153007, the tappet clearance is adjusted while the pressure in the combustion chamber that is supplied with air under high pressure is being monitored. The tappet clearance can be adjusted accurately almost without requiring any skill. According to the process described in Japanese Laid-Open Patent Publication No. 2001-27106, a point of origin for adjustment is determined from a point where the displacement of the rocker arm is reduced by a reference quantity.
The pressing lever element used by the process described in Japanese Patent Publication No. 62-8609 is complex in structure, as it needs an air microcylinder for actuation and a rotational pivot shaft as a lever mechanism. Since the pressing lever element is separate from the displacement measuring device, the apparatus is large in size.
According to the process described in Japanese Laid-Open Patent Publication No. 11-153007, because the pressure in the combustion chamber is relatively high, air flow tends to be disturbed, and hence accurate measurements cannot be made until the pressure in the combustion chamber is stabilized. Accordingly, it may be difficult to adjust the tappet clearance quickly. Furthermore, since a worker uses a screwdriver to adjust the distance at which an adjustment screw is threaded in, it is desirable to make the process automatic in order to reduce the burden on the worker, as well as to adjust the tappet clearance with higher accuracy in a shorter period of time.
According to the process described in Japanese Laid-Open Patent Publication No. 2001-27106, a worker determines the point of origin for adjustment based on displacement of the rocker arm. Consequently, the process requires a sensor for detecting displacement, and requires that a sensor signal be linked to the adjustment apparatus. The reference quantity for the determined displacement has to be converted into a rotational angle and an advanced distance, based on a relationship between the pitch and lead of the adjustment screw, and the worker has to determine a final completion point.